The present invention relates to an electrochemical sensor for determination of the oxygen content in exhaust gases.
It is generally known for determination of the oxygen content in exhaust gases, in particular in exhaust gases from an internal combustion engine, to use an electrochemical sensor.
Known sensors of this type are based on the principle of the oxygen concentration cell with an ionically conductive solid electrolyte acting as a probe element and having an inner and outer electrode. The probe element consists, for example, of a tube made of an ionically conductive solid electrolyte having an open end and a closed end. On the outer surface of the tube nearest to the open end there is an electrode which may consist, for example, of a porous platinum layer, which at the same time functions to bring about an adjustment to thermodynamic equilibrium. This adjustment of gas equilibrium is necessary, since the gas is generally not in thermodynamic equilibrium at the outset. This is a precondition for as sharp a jump in potential as possible at lambda =1 or, in the case of a polarographic limiting current probe (see German Patent 2,711,880), for the respective actual lambda value to be recorded.
The electrode layer, e.g. platinum layer, is very thin in the case of these sensors and, although it generally bears a porous ceramic protective layer, nevertheless, after prolonged use, it is subject to a corrosive attack by some of the constituents of the exhaust gas, e.g. soot, lead and phosphorous and sulfur compounds. This corrosive attack takes place over the entire area of the sensor tube, but is particularly strong in the vicinity of the open end of the tube, where, due to a lower temperature, these harmful constituents deposit more readily and do not revolatilize as readily and where the electrode layer is under certain circumstances no longer completely covered by the porous, and thus, in any case only, limitedly effective, protective layer.
To avoid the disadvantages mentioned of the known sensors, it is known from German Published Patent Application 2,619,746 to apply to the outer surface exposed to the exhaust gas of a solid electrolyte body forming the tube of the probe element an electron-conducting layer in the form of a conductor strip layer of a mixture of an electron-conducting material catalyzing the equilibration of the gas and, as needed, a ceramic material or glass, which acts as a supporting framework. This conductor strip layer extends from the closed end of the tube up to the open end. A glaze, instead of a porous ceramic cover layer of, for example, magnesium spinel or aluminum oxide, covers that portion of the conductor strip layer which is closest to the open end of the tube.
It has been found, however, that the covering of the conductor strip layer with a glaze, e.g. of potassium-aluminum silicate, barium-aluminum silicate or barium-calcium calcium-aluminum silicates, as described in German Published Patent Application 2,619,746, has a number of serious disadvantages. First, the glaze is applied only after the standard sintering process making additional work in the form of added process steps. Second, cracks readily form in this type of cover layer, so that the conductor strip layer is exposed to corrosive attack locally during operation of the sensor due to the very different material composition of the probe ceramic and thus the different thermal expansion.